Hydraulic system of work machine and work machine

ABSTRACT

A hydraulic system of a work machine includes a first hydraulic device to operate in a first operation mode while pressure of hydraulic oil supplied from a hydraulic pump via a first oil passage is equal to or higher than a first pressure threshold. The hydraulic oil in the first oil passage is to be discharged via a second oil passage. A pilot check valve is provided in the second oil passage and has a pilot port to receive a pilot pressure of the hydraulic oil. The pilot check valve is closed to stop discharging the hydraulic oil in the first oil passage through the second oil passage while the pilot pressure is lower than the fourth pressure threshold. The pilot check valve is opened while the pilot pressure is higher than or equal to the fourth pressure threshold. The first hydraulic device is a ride control device.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of the U.S. patentapplication Ser. No. 16/833,689 filed Mar. 30, 2020, which is adivisional application of the U.S. patent application Ser. No.15/369,890 filed Dec. 6, 2016, which was issued as U.S. Pat. No.10,641,388, which claims priority under 35 U. S. C. § 119 to JapanesePatent Application No. 2015-238561, filed Dec. 7, 2015, Japanese PatentApplication No. 2016-188002, filed Sep. 27, 2016. The contents of theseapplications are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a hydraulic system of a work machineand to a work machine.

Discussion of the Background

Conventionally, a hydraulic system that shifts gears by using a variabledisplacement hydraulic motor in a work machine such as a skid steerloader or a compact track loader is known (see JP 2013-36276 A).

The hydraulic system disclosed in JP 2013-36276 A is a system thatshifts gears by using a swash-plate variable displacement axial motor(HST motor). The hydraulic system has a speed changing mechanism thatcan change the speed of the HST motor. The speed changing mechanismincludes a direction switching valve, a hydraulic switching valve whoseposition is switched by the direction switching valve, and a swash-plateswitching cylinder that is connected to the hydraulic switching valveand the HST motor. The speed changing mechanism first stretches orcontracts the swash-plate switching cylinder by changing the position ofthe hydraulic switching valve by using the direction switching valve.When the swash-plate switching cylinder stretches or contracts, an angleof the swash plate of the HST motor changes, and thus the HST motor isswitched to the first speed or the second speed.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a hydraulic system ofa work machine includes a hydraulic pump, a first hydraulic device, afirst oil passage, a second oil passage, a pilot check valve, a thirdoil passage, a first actuator, a hydraulic actuator, and an operationcontrol valve. The hydraulic pump is to discharge hydraulic oil. Thefirst hydraulic device is to operate in a first operation mode whilepressure of the hydraulic oil supplied from the hydraulic oil is equalto or higher than a first pressure threshold. The first oil passageconnects the first hydraulic device and the hydraulic pump. Thehydraulic oil is to be supplied to the first hydraulic device from thehydraulic pump via the first oil passage. The second oil passage isconnected to the first oil passage. The hydraulic oil in the first oilpassage is to be discharged via the second oil passage. The pilot checkvalve is provided in the second oil passage and has a pilot port toreceive a pilot pressure of hydraulic oil. the pilot pressure iscontrolled to be a pressure lower than a fourth pressure threshold whenan operation mode of the first hydraulic device is changed to the firstoperation mode. The pilot check valve is closed to stop discharging thehydraulic oil in the first oil passage through the second oil passagewhile the pilot pressure is lower than the fourth pressure threshold.The pilot valve is opened such that the hydraulic oil in the first oilpassage is discharged through the second oil passage while the pilotpressure is higher than or equal to the fourth pressure threshold. Thethird oil passage is connected to the pilot port. The first actuator isto control an amount of hydraulic oil flowing to the third oil passage.The operation control valve is to control an operation of the hydraulicactuator. The first hydraulic device is a ride control device to performa vibration control operation to suppress a pressure fluctuation of thehydraulic actuator in accordance with the pressure of the hydraulic oil.The first actuator is a remote control valve connected to the operationcontrol valve to control the pressure of the hydraulic oil in accordancewith an operation of an operating member.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a diagram illustrating a hydraulic system (hydraulic circuit)according to a first embodiment;

FIG. 2 is a diagram illustrating a modification of the hydraulic system(hydraulic circuit) according to the first embodiment;

FIG. 3 is a diagram illustrating a hydraulic system (hydraulic circuit)according to a second embodiment;

FIG. 4 is a diagram illustrating a hydraulic system (hydraulic circuit)according to a third embodiment;

FIG. 5 is a diagram illustrating a hydraulic system (hydraulic circuit)according to a fourth embodiment;

FIG. 6 is a diagram for explaining anti-stall and illustrates arelationship between primary pressure and engine rotational speed;

FIG. 7 is a diagram illustrating a hydraulic system (hydraulic circuit)according to a fifth embodiment;

FIG. 8 is a diagram illustrating a hydraulic system (hydraulic circuit)according to a sixth embodiment;

FIG. 9 is a diagram illustrating a hydraulic system (hydraulic circuit)according to a seventh embodiment;

FIG. 10 is a diagram illustrating a hydraulic system (hydraulic circuit)according to an eighth embodiment;

FIG. 11 is a diagram illustrating a hydraulic system (hydraulic circuit)according to a ninth embodiment;

FIG. 12 is a side view illustrating a track loader that is an example ofa work machine; and

FIG. 13 is a side view illustrating part of a track loader in a statewhere a cabin is raised.

DETAILED DESCRIPTION OF EMBODIMENTS

The embodiments will now be described with reference to the accompanyingdrawings, wherein like reference numerals designate corresponding oridentical elements throughout the various drawings.

Hereinafter, embodiments of a hydraulic system of a work machine and awork machine including the hydraulic system according to the presentinvention will be described with reference to the drawings.

First Embodiment

First, an overall configuration of a work machine is described.

A work machine 1 according to the embodiments of the present inventionincludes a frame 2, a working device 3 attached to the frame 2, and atravelling device 4 that supports the frame 2, as illustrated in FIGS.12 and 13. Although a track loader is illustrated as an example of thework machine in FIGS. 12 and 13, the work machine according to theembodiments of the present invention is not limited to a track loaderand can be, for example, a tractor, a skid steer loader, a compact trackloader, or a backhoe. The embodiments of the present invention aredescribed assuming that a front side (left side in FIG. 12) of a driversitting on a driver's seat of the work machine is a forward direction, arear side (right side in FIG. 12) of the driver is a backward direction,a left side (near side in FIG. 12) of the driver is a leftwarddirection, and a right side (far side in FIG. 12) of the driver is arightward direction.

A cabin 5 is mounted in an upper front part of the frame 2. A rear partof the cabin 5 is supported by a supporting bracket 11 of the frame 2 soas to be swingable around a supporting shaft 12. A front part of thecabin 5 can be placed on a front part of the frame 2.

A driver's seat 13 is provided in the cabin 5. An operation device fortravelling 14 for operating the travelling device 4 is disposed on oneside (e.g., left side) of the driver's seat 13.

The travelling device 4 is realized by a crawler-type travelling device.The travelling devices 4 are provided below the frame 2 on the left andright of the frame 2. The travelling device 4 has a first travellingpart 21L and a second travelling part 21R that work by hydraulic drivingand is configured to be able to travel by the first travelling part 21Land the second travelling part 21R.

The working device 3 includes a boom 22L, a boom 22R, and a bucket 23(working tool). The boom 22L is disposed on the left of the frame 2. Theboom 22R is disposed on the right of the frame 2. The boom 22L and theboom 22R are linked to each other by a link. The boom 22L and the boom22R are supported by a first lift link 24 and a second lift link 25. Alift cylinder 26, which is realized by a double-acting pressurecylinder, is provided between base sides of the boom 22L and the boom22R and a lower rear part of the frame 2. The boom 22L and the boom 22Rswing up or down when the lift cylinders 26 stretch or contractconcurrently. An attachment bracket 27 is pivotably supported by a frontend side of each of the boom 22L and the boom 22R so as to be rotatableabout a horizontal shaft, and a rear surface side of the bucket 23 isattached to the attachment brackets 27 provided on the left and right.

A tilt cylinder 28, which is realized by a double-acting pressurecylinder, is provided between the attachment brackets 27 andintermediate parts of the boom 22L and the boom 22R on front end sidesthereof. The bucket 23 swings (scoops or dumps) when the tilt cylinder28 stretches or contracts.

The bucket 23 is detachably attached to the attachment brackets 27. Bydetaching the bucket 23 and attaching various attachments(hydraulically-driven working tool having a hydraulic actuator that willbe described later) to the attachment brackets 27, various works otherthan digging (or a different kind of digging operation) can beperformed. A prime mover 29 is provided on a rear side of the frame 2.The prime mover 29 is an engine. Note that the prime mover 29 may be anelectric motor or may be one that has both an engine and an electricmotor. Furthermore, a tank (hydraulic oil tank) 31 in which hydraulicoil is stored is provided in the frame 2.

Next, a hydraulic system of the work machine according to theembodiments of the present invention is described below.

FIG. 1 is an overall view of a hydraulic system of a travelling system.

As illustrated in FIG. 1, the hydraulic system (hydraulic circuit) has afirst hydraulic pump P1. The first hydraulic pump P1 is a hydraulic pumpthat is driven by power of the engine 29 to discharge hydraulic oil andis, for example, realized by a fixed displacement gear pump. The firsthydraulic pump P1 is mainly used for supply of a control signal (pilotpressure). For convenience of description, hydraulic oil discharged fromthe first hydraulic pump P1 or hydraulic oil for a control signal ishereinafter sometime referred to as pilot oil, and the pressure of pilotoil is hereinafter sometime referred to as pilot pressure. Note that thehydraulic system has a second hydraulic pump (not illustrated). Thesecond hydraulic pump is a pump that mainly supplies hydraulic oil to ahydraulic system of a work system. The lift cylinder 26, the tiltcylinder 28, and the like work by the hydraulic oil discharged from thesecond hydraulic pump.

The hydraulic system (hydraulic circuit) includes a first drivingcircuit 32A and a second driving circuit 32B. The first driving circuit32A is a circuit that drives the first travelling part 21L provided onthe left, and the second driving circuit 32B is a circuit that drivesthe second travelling part 21R provide on the right.

The first driving circuit 32A and the second driving circuit 32B eachinclude an HST pump (travelling pump) 50. The HST pumps 50 are connectedto respective travelling motors 51 of the first and second travellingparts 21L and 21R by a pair of oil passages for gear shift 71 h and 71i. Note that the second driving circuit 32B has an identical structureto the first driving circuit 32A and therefore description thereof isomitted.

The HST pump 50 is a swash-plate variable displacement axial pump thatis driven by power of the engine 29 and is a pilot-type hydraulic pump(swash-plate variable displacement hydraulic pump) in which an angle ofa swash plate is changed by pilot pressure. Specifically, the HST pump50 includes a pressure receiving part (a pilot port) for forwardtravelling 50 a and a pressure receiving part (a pilot port) forbackward travelling 50 b on which the pilot pressure acts.

The angle of the swash plate is changed by the pilot pressure that actson the pressure receiving parts 50 a and 50 b. When the angle of theswash plate changes, a direction and an amount of discharge of hydraulicoil changes, and thus rotation output of the first and second travellingparts 21L and 21R changes. When the rotational speed of the HST pump 50increases, the amount of discharge of the HST pump 50 increases, and thetravelling speed increases accordingly. The rotational speed of the HSTpump 50, i.e., the amount of discharge of the HST pump 50 changesdepending on output of the engine 29.

The hydraulic system has a first hydraulic device whose operation modecan be changed in a case where the pressure of the hydraulic oil becomesequal to or higher than a predetermined value (a first pressurethreshold). In the present embodiment, the first hydraulic device is thetravelling motor (hydraulic motor for travelling) 51 provided in each ofthe first travelling part 21L and the second travelling part 21R. Thetravelling motor 51 is a travelling motor (HST motor) whose speed can bechanged by the pressure of the hydraulic oil and works by the hydraulicoil discharged from the HST pump 50. The travelling motor 51 is, forexample, a swash-plate variable displacement axial motor that can beshifted to two speeds, i.e., high speed and low speed.

The travelling motor 51 can be operated by a travelling operation device14. The travelling operation device 14 has a remote control valve forforward travelling 36, a remote control valve for backward travelling37, a remote control valve for rightward turn 38, a remote control valvefor leftward turn 39, a travelling lever 40, which is an operatingmember, and first through fourth shuttle valves 41, 42, 43, and 44. Theremote control valves (first actuators) 36, 37, 38, and 39 are operatedby a common, i.e., single travelling lever 40. To the remote controlvalves 36, 37, 38, and 39, the hydraulic oil discharged from the firsthydraulic pump P1 is supplied. The pressure of the hydraulic oilsupplied to the remote control valves 36, 37, 38, and 39 changes inaccordance with an operation of the travelling lever (operating member)40.

The travelling lever 40 is tiltable from a neutral position in forwardand backward direction, a width direction orthogonal to the forward andbackward directions, and oblique directions. By tilting the travellinglever 40, the remote control valves 36, 37, 38, and 39 of the travellingoperation device 14 are operated. As a result, pilot pressure that isproportional to the amount of operation of the travelling lever 40 fromthe neutral position is output from secondary-side ports of the remotecontrol valves 36, 37, 38, and 39. That is, the remote control valves36, 37, 38, and 39 are realized by proportional valves. Note that theremote control valves need not be proportional valves, as long as thepressure of hydraulic oil changes in accordance with an operation of thetravelling lever (operating member) 40.

In a case where the travelling lever 40 is tilted forward (in adirection indicated by arrow A1 in FIG. 1), the remote control valve forforward travelling 36 is operated, and pilot pressure is output from theremote control valve 36. This pilot pressure acts on the pressurereceiving part for forward travelling 50 a of the first driving circuit32A after travelling from the first shuttle valve 41 through a firstoperation oil passage 75 a and acts on the pressure receiving part forforward travelling 50 a of the second driving circuit 32B aftertravelling from the second shuttle valve 42 through a second operationoil passage 75 b. As a result, output shafts 51 a of the firsttravelling part 21L and the second travelling part 21R normally rotate(rotate forwardly) at a speed that is proportional to an amount of tiltof the travelling lever 40, and thus the track loader 1 travels straightin the forward direction.

In a case where the travelling lever 40 is tilted backward (in adirection indicated by arrow A2 in FIG. 1), the remote control valve forbackward travelling 37 is operated, and pilot pressure is output fromthe remote control valve 37. This pilot pressure acts on the pressurereceiving part for backward travelling 50 b of the first driving circuit32A after travelling from the third shuttle valve 43 through a thirdoperation oil passage 75 c and acts on the pressure receiving part forbackward travelling 50 b of the second driving circuit 32B aftertravelling from the fourth shuttle valve 44 through a fourth operationoil passage 75 d. As a result, the output shafts 51 a of the firsttravelling part 21L and the second travelling part 21R reversely rotate(rotate backwardly) at a speed that is proportional to an amount of tiltof the travelling lever 40, and thus the track loader 1 travels straightin the backward direction.

In a case where the travelling lever 40 is tilted rightward (in adirection indicated by arrow A3 in FIG. 1), the remote control valve forrightward turn 38 is operated, and pilot pressure is output from theremote control valve 38. This pilot pressure acts on the pressurereceiving part for forward travelling 50 a of the first driving circuit32A after travelling from the first shuttle valve 41 through the firstoperation oil passage 75 a and acts on the pressure receiving part forbackward travelling 50 b of the second driving circuit 32B aftertravelling from the fourth shuttle valve 44 through the fourth operationoil passage 75 d. As a result, the output shaft 51 a of the firsttravelling part 21L normally rotates and the output shaft 51 a of thesecond travelling part 21R reversely rotates, and thus the track loaderturns rightward.

In a case where the travelling lever 40 is tilted leftward (in adirection indicated by arrow A4 in FIG. 1), the remote control valve forleftward turn 39 is operated, and pilot pressure is output from theremote control valve 39. This pilot pressure acts on the pressurereceiving part for forward travelling 50 a of the second driving circuit32B after travelling from the second shuttle valve 42 through the secondoperation oil passage 75 b and acts on the pressure receiving part forbackward travelling 50 b of the first driving circuit 32A aftertravelling from the third shuttle valve 43 through the third operationoil passage 75 c. As a result, the output shaft 51 a of the secondtravelling part 21R normally rotates and the output shaft 51 a of thefirst travelling part 21L reversely rotates, and thus the track loaderturns leftward.

In a case where the travelling lever 40 is tilted in an obliquedirection, directions and speeds of rotation of the output shafts 51 aof the first travelling part 21L and the second travelling part 21R aredetermined by differential pressure between pilot pressure that acts onthe pressure receiving part for forward travelling 50 a and pilotpressure that acts on the pressure receiving part for backwardtravelling 50 b of each of the first and second driving circuits 32A and32B, and thus the track loader 1 turns rightward or leftward whiletravelling in the forward or backward direction.

Specifically, in a case where the travelling lever 40 is tilted in adiagonally forward leftward direction, the track loader 1 turns leftwardwhile travelling in the forward direction at a speed corresponding to anangle of tilt of the travelling lever 40. In a case where the travellinglever 40 is tilted in a diagonally forward rightward direction, thetrack loader 1 turns rightward while travelling in the forward directionat a speed corresponding to an angle of tilt of the travelling lever 40.In a case where the travelling lever 40 is tilted in a diagonallybackward leftward direction, the track loader 1 turns leftward whiletravelling in the backward direction at a speed corresponding to anangle of tilt of the travelling lever 40. In a case where the travellinglever 40 is tilted in a diagonally backward rightward direction, thetrack loader 1 turns rightward while travelling in the backwarddirection at a speed corresponding to an angle of tilt of the travellinglever 40.

A circuit (structure) around the first travelling part 21L is describedbelow. Note that a circuit (structure) of the second travelling part 21Ris similar to that of the first travelling part 21L.

The travelling motor (first hydraulic device) 51 of the first travellingpart 21L includes a swash plate switching cylinder 53. The swash plateof the travelling motor 51 is linked to the swash plate switchingcylinder 53. The speed of the travelling motor 51 is changed bysupplying hydraulic oil to the swash plate switching cylinder 53, whichis realized by a hydraulic cylinder or the like. That is, the angle ofthe swash plate of the travelling motor 51 is changed by stretching orcontracting the swash plate switching cylinder 53. In this way, thetravelling motor 51 is shifted to the first speed or the second speed.

A first oil passage (discharge oil passage) 71 is connected to thetravelling motor 51, i.e., the swash plate switching cylinder 53. Thefirst hydraulic pump P1 is connected to the first oil passage 71.Accordingly, hydraulic oil discharged from the first hydraulic pump P1can be supplied to the travelling motor 51 (the swash plate switchingcylinder 53). The reference signs X1, X2, and X3 in FIG. 1 indicateswhere the oil passage is connected.

Specifically, the first oil passage 71 includes a first supply passage71 a, a second supply passage 71 b, a third supply passage 71 c, and afourth supply passage 71 d. The first driving circuit 32A and the seconddriving circuit 32B are connected to the first supply passage 71 a, sothat pilot oil that is discharged from the first hydraulic pump P1 canbe supplied to the first driving circuit 32A and the second drivingcircuit 32B. The travelling operation device 14 is connected to thesecond supply passage 71 b, and the pilot oil of the first hydraulicpump P1 is supplied to the travelling operation device 14 through thesecond supply passage 71 b. A first actuating part 61 and a secondactuating part 62 are connected to the third supply passage 71 c. Thepilot oil of the first hydraulic pump P1 is supplied to the firstactuating part 61 and the second actuating part 62. The fourth supplypassage 71 d connects the first hydraulic pump P1 and the swash plateswitching cylinder 53.

A second oil passage 72 is connected to the first oil passage 71. Thesecond oil passage 72 is connected to an intermediate part of the fourthsupply passage 71 d. In the fourth supply passage 71 d, a restrictor(restricting part) 80 that reduces the amount of hydraulic oil isprovided on an upstream side of a connection part where the fourthsupply passage 71 d and the second oil passage 72 are connected. Thesecond oil passage 72 is an oil passage that can discharge the hydraulicoil in the first oil passage 71. The hydraulic oil tank 31 is connectedto the second oil passage 72. A pilot check valve 65 is provided at anintermediate part of the second oil passage 72. The pilot check valve 65has a pressure receiving part (a pilot port) 65 a that receives thepressure of the hydraulic oil. In a case where the pressure of thehydraulic oil applied to the pressure receiving part 65 a is equal to orhigher than a predetermined value (a second pressure threshold), thepilot check valve 65 closes. That is, the pilot check valve 65 blocksdischarge of the hydraulic oil in the second oil passage 72 when thepressure receiving part 65 a receives the pressure of the hydraulic oil.Meanwhile, in a case where the pressure of the hydraulic oil applied tothe pressure receiving part 65 a is less than the predetermined value,the pilot check valve 65 can be opened. That is, the pilot check valve65 allows discharge of the hydraulic oil in the second oil passage 72when the pressure receiving part 65 a does not receive the pressure ofthe hydraulic oil.

When discharge of the hydraulic oil from the second oil passage 72 isblocked by the pilot check valve 65, the pressure in the first oilpassage 71 (fourth supply passage 71 d) rises. This makes it possible tostretch the swash plate switching cylinder 53 from the first speed tothe second speed. That is, the travelling motor (first hydraulic device)51 can be shifted from the first speed to the second speed by blockingdischarge of the hydraulic oil from the second oil passage 72 by usingthe pilot check valve 65. Since the swash plate switching cylinder 53 isstretched or contracted (speed is changed) by using the pilot checkvalve 65, it is possible to suppress shock at the time of speed change(at the time of gear shift). It is possible to suppress shock in a casewhere an operation mode of the travelling motor (first hydraulic device)51 is changed. Furthermore, it is possible to improve operability at thetime of speed change of the travelling motor 51.

The first actuating part 61 causes the hydraulic oil to act on thepressure receiving part 65 a of the pilot check valve 65. The pressurereceiving part 65 a of the pilot check valve 65 and the first actuatingpart 61 are connected by a third oil passage 73.

The first actuating part 61 is a two-position switch-over valve whoseposition can be switched between a first position 61 a and a secondposition 61 b. The first actuating part 61 is switched, for example, bya control device 64. An operating member 66 is connected to the controldevice 64. The operating member 66 is realized by a see-saw type switchthat is swingable, a slide-type switch that is slidable, a push-typeswitch that is capable of being pressed, or a lever.

In a case where a command to shift to the first speed is given by theoperating member 66, the control device 64 demagnetizes a solenoid ofthe first actuating part (two-position switch-over valve) 61. In a statewhere the solenoid of the first actuating part 61 is demagnetized, thefirst actuating part is at the first position 61 a, and therefore thepressure of the hydraulic oil (pilot oil) does not act on the pressurereceiving part 65 a of the pilot check valve 65. In a case where acommand to shift to the second speed is given by the operating member66, the control device 64 excites the solenoid of the first actuatingpart (two-position switch-over valve) 61. In a state where the solenoidof the first actuating part 61 is excited, the first actuating part 61is at the second position 61 b, and therefore the pressure of thehydraulic oil (pilot oil) acts on the pressure receiving part 65 a ofthe pilot check valve 65.

The hydraulic system has, separately from the travelling motor (firsthydraulic device) 51, a second hydraulic device whose operation mode canbe changed in a case where the pressure of hydraulic oil becomes equalto or higher than a predetermined value (a third pressure threshold). Inthe present embodiment, the second hydraulic device is a brakingmechanism 52 for braking the travelling motor 51.

The braking mechanism 52 is switched between a state where the brakingmechanism 52 brakes the travelling motor 51 and a state where thebraking mechanism 52 releases braking by the pilot oil (hydraulic oil)discharged from the first hydraulic pump P1. For example, the brakingmechanism 52 has a first disc provided on the output shaft 51 a of thetravelling motor 51, a second disc that is movable, and a spring thatenergizes the second disc toward a side on which the second disccontacts the first disc. Furthermore, the braking mechanism 52 includesa containing part (containing case) 52 a in which the first disc, thesecond disc, and the spring are contained. A fourth oil passage 74 isconnected to a part of the containing part 52 a in which the second discis stored. That is, the braking mechanism 52 is connected. A restrictor(restricting part) 81 is provided at an intermediate part of the fourthoil passage 74. The second actuating part 62 is connected to an upstreamside of the restricting part 81 of the fourth oil passage 74.

The second actuating part 62 causes the braking mechanism 52 to performa braking action or a braking releasing action. The second actuatingpart 62 is a two-position switch-over valve whose position can beswitched between a first position 62 a and a second position 62 b. Thesecond actuating part 62 is switched, for example, by the control device64.

The control device 64 causes the second actuating part 62 to be at thefirst position 62 a by demagnetizing a solenoid of the second actuatingpart (two-position switch-over valve) 62. In a case where the secondactuating part 62 is at the first position 62 a, the hydraulic oil inthe fourth oil passage 74 flows to the hydraulic oil tank 31. Thiscauses the pressure of the pilot oil to be equal to or lower than apredetermined value in the storage part of the containing part 52 a.Accordingly, the second disc moves toward the side on which the seconddisc contacts the first disc. The travelling motor 51 is braked by thebraking mechanism 52. The control device 64 causes the second actuatingpart 62 to be at the second position 62 b by exciting the solenoid ofthe second actuating part (two-position switch-over valve) 62. In a casewhere the second actuating part 62 is at the second position 62 b,hydraulic oil discharged from the first hydraulic pump P1 flows to thefourth oil passage 74. This causes the pressure in the storage part ofthe containing part 52 a to reach a predetermined value, thereby causingthe second disc to move to a side opposite to the case of braking (sideopposite to the side toward which the second disc is energized by thespring). Braking of the travelling motor 51 is released by the brakingmechanism 52.

In the present embodiment, the third oil passage 73 and the fourth oilpassage 74 are connected to each other. The third oil passage 73includes a first flow passage 73 a, a second flow passage 73 b, and athird flow passage 73 c. The first flow passage 73 a is an oil passagethat is connected to the fourth oil passage 74 so as to be able todischarge the hydraulic oil in the fourth oil passage 74. A restrictor(restricting part) 82 is provided at an intermediate part of the firstflow passage 73 a, and the hydraulic oil tank 31 is connected to thefirst flow passage 73 a.

The second flow passage 73 b is a flow passage that connects the firstflow passage 73 a and the first actuating part 61. One end of the secondflow passage 73 b is connected to a section of the first flow passage 73a that is located between the restricting part 82 of the first flowpassage 73 a and the hydraulic oil tank 31. A restrictor (restrictingpart) 83 is provided at an intermediate part of the second flow passage73 b, and a non-return valve (check valve) 84 is provided on an upstreamside (first actuating part 61 side) of the restricting part 83. Thecheck valve 84 allows passage of hydraulic oil that flows from the firstactuating part 61 to the downstream and blocks passage of hydraulic oilthat flows from the downstream to the first actuating part 61.

The third flow passage 73 c is a flow passage that connects the secondflow passage 73 b and the pressure receiving part 65 a. One end of thethird flow passage 73 c is connected to a section of the second flowpassage 73 b that is located between the restricting part 83 and thecheck valve 84.

According to the above arrangement, hydraulic oil (pilot oil) can besupplied to the pressure receiving part 65 a through the first flowpassage 73 a and the third flow passage 73 c in a state where braking ofthe travelling motor 51 is released by the braking mechanism 52. In acase where the first actuating part 61 is switched to the secondposition 61 b in the state where braking of the travelling motor 51 isreleased, the travelling motor 51 can be shifted from the first speed tothe second speed.

In the embodiment described above, the first oil passage 71 is connectedto the travelling motor (first hydraulic device) 51. Instead, the fourthoil passage 74 may be connected to the travelling motor (first hydraulicdevice) 51 as illustrated in FIG. 2. Specifically, the fourth oilpassage 74 has a first flow passage 74 a that connects the secondactuating part 62 and the braking mechanism (second hydraulic device) 52and a second flow passage 74 b that connects the second actuating part62 and the travelling motor (first hydraulic device) 51. In the secondflow passage 74 b, the restricting part 80, the second oil passage 72,and the swash plate switching cylinder 53 are provided. Therefore,hydraulic oil can be supplied to the travelling motor (first hydraulicdevice) 51 in a state where braking is released by the braking mechanism52 by switching the second actuating part 62 to the second position 62b. That is, in the hydraulic system of FIG. 2, the speed of thetravelling motor 51 can be changed in a case where braking is releasedand travelling is possible.

Second Embodiment

A second embodiment is a modification of the hydraulic system. FIG. 3illustrates modified parts of the hydraulic system of the secondembodiment as comparable to the first embodiment. Other parts of thesecond embodiment that are not shown in FIG. 3 are similar to those inthe first embodiment. In the description of second embodiment set forthbelow, structures that are different from that of the first embodimentare described.

As illustrated in FIG. 3, a first hydraulic device is a brakingmechanism 52 for braking the travelling motor, and a second hydraulicdevice is a travelling pump 50. A first oil passage 71 includes a firstsupply passage 71 a, a second supply passage 71 b, a fifth supplypassage 71 e, and a sixth supply passage 71 f. The fifth supply passage71 e connects a first hydraulic pump P1 and a containing part(containing case) 52 a of the braking mechanism 52. The sixth supplypassage 71 f is connected to the fifth supply passage 71 e. The sixthsupply passage 71 f is an oil passage through which hydraulic oil in thefifth supply passage 71 e can be discharged. A hydraulic oil tank 31 isconnected to the sixth supply passage 71 f.

A pilot check valve 65 having a pressure receiving part 65 a is providedat an intermediate part of the sixth supply passage 71 f.

A first actuating part is remote control valves (remote control valves36, 37, 38, and 39) that change the pressure of hydraulic oil inaccordance with an operation of a travelling lever 40. For convenienceof description, the following description takes the remote control valve36 and the remote control valve 37 as an example. Note that the remotecontrol valves are not limited to the ones illustrated in theembodiment. The first actuating part (remote control valves) and thetravelling pump (second hydraulic device) 50 are connected to each otherby a fifth oil passage 75. The fifth oil passage 75 includes a secondoperation oil passage 75 b that connects the remote control valve 36 anda pressure receiving part for forward travelling 50 a of the travellingpump 50 and a fourth operation oil passage 75 d that connects the remotecontrol valve 37 and a pressure receiving part for backward travelling50 b of the travelling pump 50.

The fifth oil passage 75 and a third oil passage 73 are connected toeach other. The third oil passage 73 has a fourth flow passage 73 d thatis connected to the second operation oil passage 75 b and a fifth flowpassage 73 e that is connected to the fourth operation oil passage 75 d.A check valve 85 is connected to an intermediate part of each of thefourth flow passage 73 d and the fifth flow passage 73 e. The fourthflow passage 73 d and the fifth flow passage 73 e merge with each other,and a merged side thereof is connected to the pressure receiving part65. Note that a high-pressure selection valve that selects a higherpressure may be used instead of the check valve 85. A bleed-off circuit(bleed-off oil passage) 88, i.e., a sixth oil passage 88 through whichhydraulic oil is discharged is provided in an oil passage into which thefourth flow passage 73 d and the fifth flow passage 73 e merge.

According to the above configuration, when the first actuating part(remote control valve) is operated, the travelling pump (secondhydraulic device) 50 works in accordance with pressure that is outputfrom the remote control valve. In this case, the pressure of hydraulicoil not only in the fifth oil passage 75, but also in the third oilpassage 73 (the fourth flow passage 73 d and the fifth flow passage 73e) rises. When the pressure of the hydraulic oil in the third oilpassage 73 rises, the pilot check valve 65 closes (blocks flow of thehydraulic oil in the sixth supply passage 71 f to the hydraulic oil tank31). As a result, the pressure of the sixth supply passage 71 fincreases, and the braking mechanism 52 releases braking. That is,according to the second embodiment, it is possible to automaticallyrelease braking in a case where a travelling operation is performed byusing the travelling lever 40.

When an operation of the first actuating part (remote control valve) isstopped, the hydraulic oil in the third oil passage 73 (the fourth flowpassage 73 d and the fifth flow passage 73 e) is discharged to thehydraulic oil tank 31 through the bleed-off circuit (bleed-off oilpassage) 88, and the pressure in the third oil passage 73 dropsaccordingly. The braking mechanism 52 performs a braking action. Thatis, it is possible to automatically perform a braking action in a casewhere the travelling operation of the travelling lever 40 is stopped.

Although the first actuating part that is capable of changing the amountof flow of hydraulic oil to the third oil passage is the remote controlvalves (remote control valves 36, 37, 38, and 39), the remote controlvalves need not be directly connected to a travelling member (travellinglever 40). Furthermore, the hydraulic system may have a structure suchthat a swash plate of the travelling pump 50 is operated directly by thepressure of hydraulic oil discharged from the remote control valves.

Although the first hydraulic device is the braking mechanism 52 and thesecond hydraulic device is the travelling pump 50 in the abovedescription, the first hydraulic device and the second hydraulic deviceare not limited to the braking mechanism 52 and the travelling pump 50.That is, the second hydraulic device can be any hydraulic device thatworks in accordance with the pressure of hydraulic oil that is outputfrom a remote control valve. This allows the first hydraulic device towork in a case where the operating member (remote control valve) thatcauses the second hydraulic device to work is operated. That is, thefirst hydraulic device and the second hydraulic device can be caused towork in conjunction with an operation of the operating member.

Third Embodiment

A third embodiment is a modification of the hydraulic system. FIG. 4illustrates modified parts of the hydraulic system of the thirdembodiment as comparable to the second embodiment. Other parts of thethird embodiment that are not shown in FIG. 4 are similar to those inthe first embodiment or the second embodiment.

As illustrated in FIG. 4, a first hydraulic device is a brakingmechanism 52, and a second hydraulic device is a travelling pump 50.Remote control valves (remote control valves 36 and 37) areelectromagnetic proportional valves whose degrees of opening are changedon the basis of a control signal that is output from a control device64. That is, in the second embodiment, an operation of the travellinglever 40 directly acts on the remote control valves, and thus the remotecontrol valves change the pressure of hydraulic oil. Meanwhile, theremote control valves in the third embodiment are valves thatelectrically act. For convenience of description, FIG. 4 illustrates theremote control valves 36 and 37. However, the following description canalso be applied to other remote control valves 38 and 39 and hydraulicdevices and the like that correspond to the remote control valves 38 and39.

An operating member 101 is connected to the control device 64. Theoperating member 101 is a travelling lever that is swingable. In a casewhere the travelling lever 101 is operated, the amount of operationand/or a direction of swing are input to the control device 64. Thecontrol device 64 supplies a control signal corresponding to the amountof swing that is output from the travelling lever 101 to a remotecontrol valve. The degree of opening of the remote control valve changesin accordance with the control signal supplied from the control device64. Therefore, the travelling pump 50 can be normally rotated orreversely rotated as in the above embodiments by operating thetravelling pump 101.

As illustrated in FIG. 4, a first switching valve 102 is connected to ableed-off circuit (sixth oil passage) 88. The switching valve 102 is atwo-position switch-over valve whose position can be switched between afirst position 102 a and a second position 102 b and is a hydraulic lockvalve for hydraulic locking. The hydraulic lock valve 102 is switched,for example, by the control device 64. A switch 103 is connected to thecontrol device 64. In a case where the switch 103 is on, the controldevice 64 demagnetizes a solenoid of the hydraulic lock valve 102. Thehydraulic lock valve 102 is at the first position 102 a in the statewhere the solenoid thereof is demagnetized. Accordingly, hydraulic oilin the third oil passage 73 (a fourth flow passage 73 d and a fifth flowpassage 73 e) is discharged to a hydraulic oil tank 31 through the sixthoil passage 88. As a result, the pressure in the third oil passage 73drops, and the braking mechanism 52 performs a braking action. That is,a braking action can be performed in conjunction with hydraulic lockingby turning the switch 103 for hydraulic locking on.

In a case where the switch 103 is off, the control device 64 excites thesolenoid of the hydraulic lock valve 102. The hydraulic lock valve 102is at the second position 102 b in the state where the solenoid thereofis excited. Accordingly, the hydraulic oil in the third oil passage 73does not flow through the sixth oil passage 88. As a result, thepressure in the third oil passage 73 rises, and the braking mechanism 52releases braking. That is, braking can be released in conjunction withhydraulic locking by turning the switch 103 off.

Fourth Embodiment

A fourth embodiment is a modification of the hydraulic system. FIG. 5illustrates modified parts of the hydraulic system of the fourthembodiment as comparable to the second embodiment. Other parts of thefourth embodiment that are not shown in FIG. 5 are similar to those inthe first embodiment or the second embodiment.

As illustrated in FIG. 5, a first hydraulic device is a brakingmechanism 52, and a second hydraulic device is a travelling pump 50. Afirst actuating part is an electromagnetic proportional valve 105 thatis provided on an upstream side of remote control valves (remote controlvalves 36 and 37). For convenience of description, the remote controlvalves 36 and 37 are illustrated in the fourth embodiment, asillustrated in FIG. 5. However, the following description can also beapplied to other remote control valves 38 and 39 and hydraulic devicesand the like that correspond to the remote control valves 38 and 39.

The electromagnetic proportional valve 105 is, for example, a controlvalve (anti-stall control valve) that prevents engine stall. The degreeof opening of the electromagnetic proportional valve 105 is changed by acontrol device 64. Anti-stall control performed by the control device 64and the electromagnetic proportional valve 105 is described below.

The control device 64 prevents engine stall by changing the degree ofopening of the electromagnetic proportional valve 105 on the basis of adrop amount of an engine, which is a difference between targetrotational speed of the engine and actual rotational speed of theengine. Note that the control device 64 is capable of acquiring theactual rotational speed of the engine and the target rotational speed ofthe engine.

FIG. 6 illustrates a relationship among engine rotational speed,travelling primary pressure, and controls lines L1 and L2.

The travelling primary pressure is pressure (pilot pressure) ofhydraulic oil in a second supply passage 71 b from the electromagneticproportional valve 105 to the remote control valves. That is, thetravelling primary pressure is primary pressure of hydraulic oil thatenters the remote control valves provided in the travelling lever 40 fora travelling operation. The control line L1 indicates a relationshipbetween the engine rotational speed and the travelling primary pressurein a case where the drop amount is less than a predetermined value. Thecontrol line L2 indicates a relationship between the engine rotationalspeed and the travelling primary pressure in a case where the dropamount is equal to or larger than the predetermined value.

In a case where the drop amount is less than the predetermined value,the control device 64 adjusts the degree of opening of theelectromagnetic proportional valve 105 so that the relationship betweenthe actual rotational speed of the engine and the travelling primarypressure coincides with the control line L1. In a case where the dropamount is equal to or larger than the predetermined value, the controldevice 64 adjusts the degree of opening of the electromagneticproportional valve 105 so that the relationship between the actualrotational speed of the engine and the travelling primary pressurecoincides with the control line L2. On the control line L2, thetravelling primary pressure with respect to predetermined enginerotational speed is lower than that on the control line L1. That is, thetravelling primary pressure of the control line L2 is lower than that ofthe control line L1 with respect to the same engine rotational speed.Accordingly, the pressure (pilot pressure) of the hydraulic oil thatenters the remote control valves is kept low by control based on thecontrol line L2. As a result, a swash plate of an HST pump 50 isadjusted, and load that acts on an engine 29 decreases. It is thereforepossible to prevent stall of the engine 29. Note that although a singlecontrol line L2 is illustrated in FIG. 6, the number of control lines L2may be more than one. For example, the control line L2 may be set foreach engine rotational speed.

It is preferable that the control device 64 has data indicative of thecontrol line L1 and the control line L2, control parameters of afunction or the like.

A third oil passage 73 is connected to a section 71 b 1 of the secondsupply passage 71 b that is located between the electromagneticproportional valve 105 and the remote control valves. With thearrangement, since the pressure in the third oil passage 73 rises whilethe electromagnetic proportional valve 105 is in operation, braking canbe released in conjunction with anti-stall control. Note that althoughan anti-stall control valve is used as an example of the electromagneticproportional valve 105 in the present embodiment, the electromagneticproportional valve 105 need not be an anti-stall control valve.

Fifth Embodiment

A fifth embodiment is an embodiment in which a pilot check valve 110 isused instead of the pilot check valve 65, as illustrated in FIG. 7.

The pilot check valve 110 has a pressure receiving part (a pilot port)110 a that receives the pressure of hydraulic oil. In a case where thepressure of the hydraulic oil applied to the pressure receiving part 110a is equal to or higher than a predetermined value (a fourth pressurethreshold), the pilot check valve 110 opens. That is, the pilot checkvalve 110 allows discharge of the hydraulic oil in a second oil passagewhen the pressure receiving part 110 a receives the pressure of thehydraulic oil. Meanwhile, in a case where the pressure of the hydraulicoil applied to the pressure receiving part 110 a is less than thepredetermined value (the fourth pressure threshold), the pilot checkvalve 110 closes. That is, the pilot check valve 110 blocks discharge ofthe hydraulic oil in the second oil passage when the pressure receivingpart 110 a does not receive the pressure of the hydraulic oil. In thepresent embodiment, the sixth supply passage 71 f described in the firstembodiment is the second oil passage.

The pilot check valve 110 is provided at an intermediate part of thesecond oil passage (sixth supply passage 71 f), as illustrated in FIG.7. A relief valve 111 is provided on a downstream side of the pilotcheck valve 110. A first hydraulic device is a braking mechanism 52. Afirst oil passage 71 has a fifth supply passage 71 e that connects afirst hydraulic pump P1 and a containing part (containing case) 52 a ofthe braking mechanism 52 and a second supply passage 71 b connected tothe fifth supply passage 71 e. A braking valve 62 for braking or releaseof braking of the braking mechanism 52 is provided at an intermediatepart of the fifth supply passage 71 e. Note that the braking valve 62 issimilar to the second actuating part of the first embodiment.

On an upstream side of the braking valve 62, the fifth supply passage 71e is branched, and a switching valve (second switching valve) 113 isconnected to a branched oil passage 112. A third oil passage 73 that isconnected to the pilot check valve 110 is connected to the switchingvalve 113. The switching valve 113 is a two-position switch-over valvewhose position can be switched between a first position and a secondposition.

The switching valve 113 can be switched by a control device 64 or thelike. For example, a switch or the like is connected to the controldevice 64, and in a case where the switch is turned on, the controldevice 64 excites a solenoid of the switching valve 113 so that theswitching valve 113 is switched to the second position. In a case wherethe switch is turned off, the control device 64 demagnetizes thesolenoid of the switching valve 113 so that the switching valve 113 isswitched to the first position. Therefore, the pressure of hydraulic oilthat acts on the pilot check valve 110 (a flow amount of hydraulic oilflowing through the third oil passage 73) can be changed by operatingthe switch connected to the control device 64. As a result, primarypressure of hydraulic oil that acts on remote control valves (remotecontrol valve 36, 37) of a travelling operation device 14 can bechanged. For convenience of description, the remote control valves 36and 37 are illustrated in the fifth embodiment, as illustrated in FIG.7. However, the above description can also be applied to other remotecontrol valves 38 and 39 and hydraulic devices and the like thatcorrespond to the remote control valves 38 and 39.

Sixth Embodiment

A sixth embodiment is a modification of the hydraulic system. FIG. 8illustrates a case where a pilot check valve 110 is applied to ahydraulic system of a work system. First, the hydraulic system of thework system is described. Note that the hydraulic system of the worksystem is not limited to a configuration described below.

As illustrated in FIG. 8, the hydraulic system of the work system is asystem for actuating a boom 22L, a boom 22R, a bucket 23 (working tool),and the like and includes a plurality of operation control valves 115and a hydraulic pump (second hydraulic pump) P2 of the work system.

The second hydraulic pump P2 is a pump that is provided at a differentposition from the first hydraulic pump P1 and is realized by a fixeddisplacement gear pump. The second hydraulic pump P2 can dischargehydraulic oil stored in a hydraulic oil tank 22. In particular, thesecond hydraulic pump P2 mainly discharges hydraulic oil for actuating ahydraulic actuator.

A main oil passage (oil passage) 116 is provided on a discharge side ofthe second hydraulic pump P2. The plurality of operation control valves115 are connected to the main oil passage 116. The operation controlvalves 115 are valves that can change a direction of flow of hydraulicoil by pilot pressure of pilot oil.

The plurality of operation control valves 115 include a first operationcontrol valve 115A and a second operation control valve 115B. The firstoperation control valve 115A is a valve that controls a lift cylinder26. The second operation control valve 115B is a valve that controls atilt cylinder 28.

The first operation control valve 115A and the second operation controlvalve 115B are pilot-type direct-acting spool type three-positionswitch-over valves. The first operation control valve 115A and thesecond operation control valve 115B are switched among a first position,a neutral position, and a second position by the pilot pressure.

The lift cylinder 26 is connected to the first operation control valve115A via oil passages 151 and 152, and the tilt cylinder 28 is connectedto the second operation control valve 115B via oil passages.

The boom 22L, the boom 22R, and the bucket 23 can be operated by a workoperation device 119 that is provided around a driver's seat 13. Thework operation device 119 includes an operating member 120 and aplurality of remote control valves 121, 122, 123, and 124. The operatingmember 120 is an operation lever that is supported so as to be tiltablefrom a neutral position in forward, backward, leftward, rightward, andoblique directions. By tilting the operation lever 120, the plurality ofremote control valves 121, 122, 123, and 124 that are provided below theoperation lever 120 can be operated. The remote control valves 121, 122,123, and 124 and the first hydraulic pump P1 are connected to each otherby a discharge oil passage 71.

The plurality of remote control valves 121, 122, 123, and 124 and theplurality of operation control valves 115 are connected to each other bya plurality of oil passages 125 a, 125 b, 125 c, and 125 d.Specifically, the remote control valve 121 is connected to the firstoperation control valve 115A via the first operation oil passage 125 a.The remote control valve 122 is connected to the first operation controlvalve 115A via the second operation oil passage 125 b. The remotecontrol valve 123 is connected to the second operation control valve115B via the third operation oil passage 125 c. The remote control valve124 is connected to the second operation control valve 115B via thefourth operation oil passage 125 d. Each of the remote control valves121, 122, 123, and 124 can set the pressure of output hydraulic oil inaccordance with an operation of the operation lever 120.

Specifically, when the operation lever 120 is tilted forward, the remotecontrol valve for downward movement 121 is operated, and pilot pressureof pilot oil that is output from the remote control valve for downwardmovement 121 is set. This pilot pressure acts on a pressure receivingpart (a pilot port) of the first operation control valve 115A. As aresult, the lift cylinder 26 contracts, and the booms 22L and 22R movedownward.

When the operation lever 120 is tilted backward, the remote controlvalve for upward movement 122 is operated, and pilot pressure of pilotoil that is output from the remote control valve for upward movement 122is set. This pilot pressure acts on the pressure receiving part of thefirst operation control valve 115A. As a result, the lift cylinder 26stretches, and the booms 22L and 22R move upward.

When the operation lever 120 is tilted rightward, the remote controlvalve for bucket dumping 123 is operated, and pilot pressure of pilotoil that is output from the remote control valve 123 is set. This pilotpressure acts on a pressure receiving part (a pilot port) of the secondoperation control valve 115B. As a result, the tilt cylinder 28stretches, and the bucket 23 performs a dumping action.

When the operation lever 120 is tilted leftward, the remote controlvalve for bucket scooping 124 is operated, and pilot pressure of pilotoil that is output from the remote control valve 124 is set. This pilotpressure acts on the pressure receiving part of the second operationcontrol valve 115B. As a result, the tilt cylinder 28 contracts, and thebucket 23 performs a scooping action.

A float control device 130 that can bring the lift cylinder 26 into afloat state is provided as illustrated in FIG. 8. The float controldevice 130 includes a first float control valve 131, a second floatcontrol valve 132, and a float switching valve 133.

Each of the first float control valve 131 and the second float controlvalve 132 is a two-position switch-over valve whose position can beswitched between a first position where passage of hydraulic oil isblocked and a second position where passage of hydraulic oil is allowed.The first float control valve 131 and the second float control valve 132are connected to the float switching valve 133 via an oil passage 141.The first float control valve 131 is connected to the oil passage 151that connects a bottom side of the lift cylinder 26 and the firstoperation control valve 115A. Furthermore, the first float control valve131 is connected to an oil passage 153 that discharges hydraulic oil. Ina case where the first float control valve 131 is at the secondposition, the oil passage 151 and the oil passage 153 are communicatedwith each other, whereas in a case where the first float control valve131 is at the first position, communication between the oil passage 151and the oil passage 153 is blocked.

The second float control valve 132 is connected to the oil passage 152that connects a rod side of the lift cylinder 26 and the first operationcontrol valve 115A. Furthermore, the second float control valve 132 isconnected to the oil passage 153. In a case where the second floatcontrol valve 132 is at the second position, the oil passage 152 and theoil passage 153 are communicated with each other, whereas in a casewhere the second float control valve 132 is at the first position,communication between the oil passage 152 and the oil passage 153 isblocked.

Therefore, by placing the first float control valve 131 and the secondfloat control valve 132 at the second position, the bottom side and therod side of the lift cylinder 26 are communicated with the oil passage(discharge oil passage) 153, and therefore the lift cylinder 26 can bebrought into a float state. By placing the first float control valve 131and the second float control valve 132 at the first position, the liftcylinder 26 is brought into a state that is not the float state, andthus the lift cylinder 26 can be moved upward or downward by the firstoperation control valve 115A.

The first float control valve 131 and the second float control valve 132can be switched by the float switching valve 133 and the control device64. The float switching valve 133 is a two-position switch-over valvewhose position can be switched between a first position and a secondposition. For example, a switch or the like is connected to the controldevice 64, and in a case where the switch is turned on, the controldevice 64 excites a solenoid of the float switching valve 133 so thatthe float switching valve 133 is switched to the second position. Thiscauses the pressure of hydraulic oil discharged from the first hydraulicpump P1 to be output to the oil passage 141. In this way, the firstfloat control valve 131 and the second float control valve 132 can beswitched to the second position. Meanwhile, in a case where the switchis turned off, the control device 64 demagnetizes the solenoid of thefloat switching valve 133 so that the float switching valve 133 isswitched to the first position, and thus hydraulic oil in the oilpassage 141 is discharged to the hydraulic oil tank 31 or the like. Inthis way, the first float control valve 131 and the second float controlvalve 132 can be switched to the first position.

As illustrated in FIG. 8, a first actuating part that causes hydraulicoil to act on the pressure receiving part 110 a of the pilot check valve110 is the float switching valve 133. A first hydraulic device is thefirst operation control valve 115A for actuating the lift cylinder 26,and a first oil passage includes the discharge oil passage 71 and thefirst operation oil passage 125 a.

A second oil passage 72 is connected to the first operation oil passage125 a that connects the remote control valve 121 and the first operationcontrol valve 115A. That is, the second oil passage 72 is connected tothe first operation oil passage (lowering operation oil passage) 125 afor giving a command to perform an operation of lowering the liftcylinder 26 to the operation control valve 115A. The pilot check valve110 is provided in this second oil passage 72. A third oil passage 73that is connected to the pressure receiving part 110 a of the pilotcheck valve 110 is connected to the oil passage 141.

Therefore, during the float operation, hydraulic oil acts on thepressure receiving part 110 a of the pilot check valve 110. That is, ina case where an operation of lowering the booms 22L and 22R is performedduring the float operation, it is possible to prevent the operationcontrol valve 115A from working. It is therefore possible to eliminatewasteful use of hydraulic oil.

Note that the first hydraulic device may be the second operation controlvalve 115B that controls a hydraulic actuator such as the tilt cylinder28 or may be a different operation control valve. The first oil passageis applicable not only to the first operation oil passage 125 a, butalso to any of the second operation oil passage 125 b, the thirdoperation oil passage 125 c, and the fourth operation oil passage 125 d.In this case, the second oil passage 72 is also applicable to any of thesecond operation oil passage 125 b, the third operation oil passage 125c, and the fourth operation oil passage 125 d.

Seventh Embodiment

A seventh embodiment is a modification of the hydraulic system. FIG. 9illustrates modified parts of the hydraulic system of the seventhembodiment as comparable to the sixth embodiment. Other parts of theseventh embodiment that are not shown in FIG. 9 are similar to those inthe sixth embodiment.

As illustrated in FIG. 9, a plurality of second oil passages 72 areconnected to a first oil passage (first operation oil passage) 125 a. Inthe present embodiment, two second oil passages 72 are connected to thefirst operation oil passage 125 a.

Of the two second oil passages 72, a pilot check valve 110 is connectedto one second oil passage 72 a, and a pilot check valve 110 is alsoconnected to the other second oil passage 72 b. A check valve 160 thatcan change set pressure (differential pressure) is provided in thesecond oil passage 72 a.

For convenience of description, the pilot check valve 110 that isconnected to the second oil passage 72 a is hereinafter referred to as afirst pilot check valve 110A, and the pilot check valve 110 that isconnected to the second oil passage 72 b is hereinafter referred to as asecond pilot check valve 110B. Furthermore, a third oil passage 73 thatis connected to the first pilot check valve 110A is hereinafter referredto as an oil passage 73 a, and a third oil passage 73 that is connectedto the second pilot check valve 110B is hereinafter referred to as anoil passage 73 b.

A switching valve 161 is connected to the oil passage 73 a, and aswitching valve 162 is connected to the oil passage 73 b. A fourthswitching valve (the switching valve 161 and the switching valve 162) isconnected to a discharge oil passage 71 that is connected to a firsthydraulic pump P1. Each of the switching valve 161 and the switchingvalve 162 is a two-position switch-over valve whose position can beswitched between a first position and a second position. Therefore, byswitching the switching valve 161 and the switching valve 162, thepressure of hydraulic oil that acts on the first pilot check valve 110Aand the second pilot check valve 110B (a flow amount of hydraulic oilflowing through the third oil passage 73) can be changed. That is, theswitching valve 161 and the switching valve 162 are a first actuatingpart that can change the pressure of the hydraulic oil that acts on thepilot check valves 110.

The first actuating part (the switching valve 161 and the switchingvalve 162) can be switched by a control device 64 or the like. Forexample, a switch or the like that can be switched among three positionsis connected to the control device 64. In a case where the switch is ata first position, the control device 64 excites a solenoid of theswitching valve 161 and demagnetizes a solenoid of the switching valve162. This causes the switching valve 161 to be at the second positionand causes the switching valve 162 to be at the first position, andtherefore part of hydraulic oil in the first operation oil passage 125 acan be discharged by the first pilot check valve 110A. In a case wherethe switch is at a second position, the control device 64 excites thesolenoid of the switching valve 162 and demagnetizes the solenoid of theswitching valve 161. This causes the switching valve 161 to be at thefirst position and causes the switching valve 162 to be at the secondposition, and therefore part of the hydraulic oil in the first operationoil passage 125 a can be discharged by the second pilot check valve110B. In a case where the switch is at a third position, the switchingvalve 161 and the switching valve 162 are at the first position.

Therefore, the pressure of the hydraulic oil that acts on the firstoperation oil passage 125 a (a flow amount of hydraulic oil flowingthrough the third oil passage 73) can be changed by the first pilotcheck valve 110A, the second pilot check valve 110B, and the check valve160. Note that the number of pilot check valves 110, the number of checkvalves 160, and the number of switching valves 161 and 162 are notlimited to the ones described in the embodiment. Although the pluralityof second oil passages 72 are provided in the first operation oilpassage 125 a, the second oil passages 72 may be provided in a differentoil passage.

Eighth Embodiment

An eighth embodiment is a modification of the hydraulic system. FIG. 10illustrates modified parts of the hydraulic system of the eightembodiment as comparable to the sixth embodiment. Other parts of theeight embodiment that are not shown in FIG. 10 are similar to those inthe sixth embodiment.

As illustrated in FIG. 10, a first oil passage 71 includes a seventhsupply passage 71 g that connects a ride control device 170 and a firsthydraulic pump P1. The ride control device 170 is a device that performsa vibration control operation for suppressing a pressure fluctuation ofa hydraulic actuator. In the present embodiment, the ride control device170 is a device that controls vibration of a lift cylinder 26 (a boom22L and a boom 22R). The ride control device 170 includes a vibrationcontrol switching valve 171 and a vibration control part 172 thatperforms a vibration control operation. The vibration control switchingvalve 171 is a two-position switch-over valve whose position can beswitched between a first position and a second position. The vibrationcontrol switching valve 171 can be switched by a control device 64 orthe like. For example, a switch or the like is connected to the controldevice 64, and in a case where the switch is turned on, the controldevice 64 excites a solenoid of the vibration control switching valve171 so that the vibration control switching valve 171 is switched to thesecond position. In a case where the switch is turned off, the controldevice 64 demagnetizes the solenoid of the vibration control switchingvalve 171 so that the vibration control switching valve 171 is switchedto the first position. In a case where the vibration control switchingvalve 171 is at the second position, hydraulic oil acts on the vibrationcontrol part 172, thereby causing the vibration control part 172 tocontrol vibration of the lift cylinder 26. In a case where the vibrationcontrol switching valve 171 is at the first position, hydraulic oil doesnot act on the vibration control part 172, and therefore vibrationcontrol of the lift cylinder 26 by the vibration control part 172 stops.

A second oil passage 72 is connected to the seventh supply passage 71 g.A pilot check valve 110 is connected to an intermediate part of thesecond oil passage 72. A third oil passage 73 is connected to the pilotcheck valve 110. The third oil passage 73 has a sixth flow passage 73 fthat is connected to a first operation oil passage 125 a and a seventhflow passage 73 g that is connected to a second operation oil passage125 b. A check valve 173 is connected to intermediate parts of the sixthflow passage 73 f and the seventh flow passage 73 g. The sixth flowpassage 73 f and the seventh flow passage 73 g merge with each other,and a merged side thereof is connected to a pressure receiving part 110a. Note that a high-pressure selection valve that selects a higherpressure may be used instead of the check valve 173.

In the eighth embodiment, a first hydraulic device is the ride controldevice 170, and a first operating valve is remote control valves 121 and122. Therefore, an operation of controlling vibration of the liftcylinder 26 can be performed by switching the vibration controlswitching valve 171, and the vibration control operation can be releasedin a case where an operation lever 120 is operated.

Ninth Embodiment

A ninth embodiment is a modification of the hydraulic system. FIG. 11illustrates modified parts of the hydraulic system of the ninthembodiment as comparable to the ninth embodiment. Other parts of theninth embodiment that are not shown in FIG. 11 are similar to those inthe above embodiments and the like.

As illustrated in FIG. 11, the hydraulic system includes an operationdevice 181 that can perform both a travelling operation and a workoperation by a single operating member 180. The operation device 181 hasa remote control valve for forward travelling 36, a remote control valvefor backward travelling 37, a remote control valve for downward movement121, and a remote control valve for upward movement 122. The remotecontrol valve 36 and the remote control valve 37 are connected to an HSTpump 50, and the remote control valve 121 and the remote control valve122 are connected to a first operation control valve 115.

Specifically, the remote control valve 36 is connected to the HST pump50 via a first operation oil passage 75 a. The remote control valve 37is connected to the HST pump 50 via a third operation oil passage 75 c.The remote control valve 121 is connected to a first operation controlvalve 115A via a first operation oil passage 125 a. The remote controlvalve 122 is connected to the first operation control valve 115A via asecond operation oil passage 125 b. For convenience of description,operation oil passages of a travelling system (the first operation oilpassage 75 a and the third operation oil passage 75 c) are hereinafterreferred to as a first travelling oil passage 75 a and a secondtravelling oil passage 75 c.

A second oil passage 72 is connected to the first travelling oil passage75 a and the second travelling oil passage 75 c. The second oil passage72 has an oil passage 72 c that is connected to the first travelling oilpassage 75 a, an oil passage 72 d that is connected to the secondtravelling oil passage 75 c, and an oil passage 72 e into which the oilpassage 72 c and the oil passage 72 d merge. A check valve 183 isprovided in the oil passage 72 c and the oil passage 72 d.

A pilot check valve 110 is connected to the oil passage 72 e. Aswitching valve (third switching valve) 182 is connected to a third oilpassage 73 that is connected to the pilot check valve 110. The switchingvalve 182 is a two-position switch-over valve whose position can beswitched between a first position and a second position. Therefore, thepressure of the hydraulic oil that acts on the pilot check valve 110 canbe changed by switching the switching valve 182. The switching valve 182is a first actuating part.

The switching valve 182 can be switched by a control device 64 or thelike. For example, a switch or the like is connected to the controldevice 64. In a case where the switch is turned on, the control device64 excites a solenoid of the switching valve 182. This causes theswitching valve 182 to be at the second position, and therefore part ofhydraulic oil in the operation oil passages of the travelling system(the first operation oil passage 75 a and the third operation oilpassage 75 c) can be discharged by the first pilot check valve 110. In acase where the switch is turned off, the control device 64 demagnetizesthe solenoid of the switching valve 182. In this way, the HST pump 50can be operated as usual by an operation of the operating member 180.

The embodiments disclosed herein are given only for illustration andshould not be construed as being restrictive. The scope of the presentinvention is indicated not by the above description but by the claims,and it is intended that meanings equivalent to the scope of the claimsand all changes within the scope are encompassed within the embodimentsof the present invention. In the above embodiments, the first actuatingpart 61 and the second actuating part 62 are two-position switch-overvalves. Instead, the first actuating part 61 and the second actuatingpart 62 may be proportional valves. In the above embodiments, thetravelling motor 51 and the braking mechanism 52 are described ashydraulic devices. However, the hydraulic devices may be any devicesthat work by the pressure of hydraulic oil. In the above embodiments, atravelling motor is a motor that is switched between the first speed andthe second speed (motor whose travelling state is changed). However, thestages between which the travelling motor is switched is not limited tothe first speed and the second speed. In the above embodiments,hydraulic oil is discharged to a hydraulic oil tank. However, hydraulicoil may be discharged to a different place. That is, an oil passage fordischarging hydraulic oil may be connected to a place other than ahydraulic oil tank. For example, an oil passage for discharginghydraulic oil may be connected to a sucking part of a hydraulic pump(part that sucks in hydraulic oil) or may be connected to a differentpart.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed is:
 1. A hydraulic system of a work machine, comprising:a hydraulic pump to discharge hydraulic oil; a first hydraulic device tooperate in a first operation mode while pressure of the hydraulic oilsupplied from the hydraulic oil is equal to or higher than a firstpressure threshold; a first oil passage which connects the firsthydraulic device and the hydraulic pump and via which the hydraulic oilis to be supplied to the first hydraulic device from the hydraulic pump;a second oil passage which is connected to the first oil passage and viawhich the hydraulic oil in the first oil passage is to be discharged; apilot check valve provided in the second oil passage and having a pilotport to receive a pilot pressure of hydraulic oil, the pilot pressurebeing controlled to be a pressure lower than a fourth pressure thresholdwhen an operation mode of the first hydraulic device is changed to thefirst operation mode, the pilot check valve being closed to stopdischarging the hydraulic oil in the first oil passage through thesecond oil passage while the pilot pressure is lower than the fourthpressure threshold, the pilot valve being opened such that the hydraulicoil in the first oil passage is discharged through the second oilpassage while the pilot pressure is higher than or equal to the fourthpressure threshold; a third oil passage connected to the pilot port; afirst actuator to control an amount of hydraulic oil flowing to thethird oil passage; a hydraulic actuator; and an operation control valveto control an operation of the hydraulic actuator, wherein the firsthydraulic device is a ride control device to perform a vibration controloperation to suppress a pressure fluctuation of the hydraulic actuatorin accordance with the pressure of the hydraulic oil, and wherein thefirst actuator is a remote control valve connected to the operationcontrol valve to control the pressure of the hydraulic oil in accordancewith an operation of an operating member.
 2. A work machine comprising ahydraulic system according to claim 1.